ONE MIDSUMMER'S NIGHT, a student at the University of Helsinki posted a query to the newsgroup comp.os.minix asking, "What would you like to see most in minix?" The student's name was Linus Torvalds, and that Usenet post was the beginning of the Linux operating system (OS). The date was 25 August 1991, exactly 20 years ago today.
In 1991 Unix had existed for about 20 years since the early 1970s, Apple had come out with its Mac OS in 1984, and Microsoft had been flogging Windows since 1985. Torvalds' ambitions for his "new (free) operating system" were modest. It was to be "just a hobby, won't be big and professional like gnu" for IBM PC "386(486) AT clones". He wanted to call his OS kernel "Freax", but a friend who ran the FTP server that hosted the software named Torvalds' source code download directory "linux" and the name stuck.
Though at first developed under Minix, Linux soon came to be built using the free GNU software development toolchain, compiler, bash command shell and userspace commands and utilities that Richard Stallman began developing in 1984. For that reason Stallman and the Free Software Foundation prefer to call it GNU/Linux, and they have a point, but most people just call it Linux. It is licensed as free software under the GNU General Public Licence, version 2 (GPLv2) and Linus Torvalds holds the trademark on Linux.
As a free, Unix-like OS that mostly adheres to the POSIX, SUS, ISO and ANSI standards and is built using free and open source software that is based on copyleft principles, Linux quickly gained software developer interest and developed rapidly into a fully fledged OS supporting a robust software development ecosystem and thousands of software applications.
By now, 20 years later, Linux includes source code contributed over the years by thousands of software developers from all around the world. Torvalds' adoption of copyleft principles has been crucial to Linux success.
Based upon authors' legal rights under international copyright law, copyleft works a lot like the old parable of Stone Soup - everyone contributes a little bit, all share in the results, and everyone benefits.
Suit-wearing business people are more comfortable using the phrase 'Open Source' than 'Free Software', and prefer to talk about the 'Open Source Method' rather than 'Copyleft', but fundamentally these are just the same things under different names. Whatever you call this, copyleft development or the open source method, it is the factor that has driven the extemely rapid development and astonishingly pervasive adoption of Linux over the past 20 years.
Linux was the first prominent software success of the open source method, though many other software development projects have since adopted it, most notably the other major components of the LAMP stack that now runs most of the world wide web.